The present invention relates to a press contact type semiconductor device, particularly, to a press contact type semiconductor device capable of ensuring a uniform contact condition between semiconductor elements and package electrodes, and of decreasing thermal resistance and electrical resistance, and a converter using the same.
Technology of power electronics, wherein a main circuit current is controlled by using technology of semiconductor electronics, has been applied widely to various fields, and, furthermore, the applied fields are still expanding. Examples of semiconductor elements for power electronics are as follows: thyristor, light-triggered thyristor, gate-turn-off thyristor (GTO), insulated gate bipolar transistor (hereinafter, called IGBT) which is a MOS controlled device, metal oxide semiconductor field effect transistor (hereinafter, called MOSFET), and others. In these devices, main electrodes (cathode, emitter electrode) are formed on a first main plane of a semiconductor chip, and other main electrodes (anode, collector electrode) are formed on a second main plane of the semiconductor chip.
In the semiconductor devices for large power such as a GTO, light-triggered thyristor, and the like, the semiconductor element is packaged per every wafer. Both main electrodes of the above element have a structure wherein the main electrodes are contacted by pressing with a pair of external main electrode plates in the package via an intermediate electrode plate (buffer electrode plate for thermal stress) made of Mo or W. In order to improve uniformity of switching operation, large current turn-off capability, and the like, it is important to make contact conditions among the element electrodes, the intermediate electrode plates, and the external main electrode plates as uniform as possible, and to decrease contact thermal resistance and electrical resistance. Therefore, a countermeasure to decrease warps and waviness by improving precision (flatness, plainness) in manufacturing parts of the package has been generally adopted.
On the other hand, a plurality of chips have been mounted mainly by a package composition of wire-connecting electrodes in a type called a module type structure in IGBTs and the like. In case of the module type package, heat generated in the internal portion of the element chip is released from only one side of the package (a plane which has not been connected by wire), that is, the side of the package where the electrode has been mounted directly on a base substrate. Accordingly, thermal resistance is generally significant, and the number of chips which can be mounted on a package, and a usable current capacity (an amount of heat generation, or a mounting density) were limited.
Currently, in order to solve the above problems and to respond to a demand for increasing the capacity, a semiconductor device having a press contacting structure of multichips in parallel, wherein a plurality of IGBT chips are assembled in parallel in a flat type package so as to be capable of taking out an emitter electrode and collector electrode formed on the main planes of each chips by plane-contacting to a pair of external main electrode plates provided at the package, such as disclosed in JP-A-8-88240 (1996) has been receiving attention. In accordance with the semiconductor device having a press contacting structure of multichips in parallel, variations in height of each position of every chip caused by variation in dimensions of the members (parts), and variations in warps and waviness of the main electrode plate per location can not be avoided. Accordingly, the pressure varies per chip, and uniform contacts can not be obtained. Then, the thermal resistance and the electrical resistance per position of every chip varied remarkably, and a serious problem that the characteristics of the elements were unstable as a whole was created. The problem can be solved by the simple step of using members having severely precise dimensions. However, the above countermeasure can not be deemed as realistic, because increasing a production cost and a selection cost of the members can not be avoided. For solving the above problem, JP-A-8-88240 (1996) discloses a method to insert a soft metallic sheet having a ductility such as silver as a thickness correcting plate.
The size of the element (wafer size) in the package of the GTO and the like will be increased for responding to the demand for increasing their capacities in the future. Accordingly, the warps, waviness, and the like of the package members (electrode members) are in a trend to be increased in accordance with increasing the diameter of the element. The countermeasure to decrease warps and waviness by improving precision in manufacturing parts of the package (flatness, plainness) as described previously has a limit in manufacturing, and a serious problem in cost. Accordingly, it becomes increasingly difficult to ensure a uniform contact between the wafer and the package members (electrodes) in the whole plane of the element size (wafer size), and to decrease the thermal resistance and the electrical resistance.
On the other hand, in accordance with the method of inserting a soft metallic sheet disclosed as a countermeasure for solving the problem of uniform contact between chips of the semiconductor device having a press contact structure of multichips in parallel as described previously, it has been revealed by study of the inventors of the present invention that the amount of deformation of the sheet is very little (deforming only by elastic deformation) with a pressure in the range of practical use, i.e. the pressure which does not destroy at least the semiconductor chip, and the amount of deformation is not sufficient for ensuring uniform contact when the variation in height per position of every chip (and the height including the intermediate electrode members interposing the chip, and others) is remarkable.
The reason can be estimated that, when a pressure is added to a soft metallic sheet in a thickness direction to cause a plastic deformation in a lateral direction as indicated schematically in FIG. 25, deformation resistance in the lateral direction becomes extremely large, even if the soft metallic material is used, due to a friction force (friction resistance) 56 generated at the boundary between the electrode members 54, 55 interposing the soft metallic sheet 53. Even if the pressure is increased in order to cause a plastic deformation, the friction force is increased in proportion to the pressure, and the plastic deformation can not be readily caused. Particularly, in a case of a sheet form, wherein the area receiving the resistance is remarkably larger than the thickness, influence of the friction force generated on the surface of the area becomes dominant, and even if a pressure exceeding the yield stress of the material known to the public is added, substantial plastic deformation (flow) is not caused practically, and the thickness of the soft metallic sheet is scarcely changed before and after pressing.
The present invention is to provide a method for ensuring a uniform press contact condition on a large area region, which has been becoming more difficult than ever in accordance with increasing the size of the package by increasing the diameter of the wafer, and with connecting multichips in parallel of the element corresponding to increasing the capacity. That is, the present invention is to provide a method which is capable of accommodating the variation in height of the contacting plane (warps, waviness, variation in dimensions of the members, and the like), and of decreasing the thermal resistance and electrical resistance at the contacting boundary planes. The second object of the present invention is to provide a converter preferable for a system of a large capacity by using the semiconductor device obtained by the above methods.
In accordance with the press contact type semiconductor device relating to the present invention, a semiconductor element comprising at least a first main electrode on a first main plane, and a second main electrode on a second main plane is assembled between a pair of main electrode plates, and a metallic body having macroscopic vacancies inside is arranged between electrodes of said semiconductor element and the main electrode plate.
Furthermore, in accordance with the press contact type semiconductor device having a practical composition relating to the present invention, a semiconductor element comprising at least a first main electrode on a first main plane, and a second main electrode on a second main plane is assembled between a pair of main electrode plates, and any one selected from at least a metallic netting, a plate manufactured to be uneven, and a porous metallic plate is arranged between electrodes of said semiconductor element and the main electrode plate.